Power Supply Equalization Circuit Using Distributed High-Voltage and Low-Voltage Shunt Circuits

ABSTRACT

Embodiments of an IC protection circuit that protects low voltage supply transistors and circuits within the IC from excessive power supply levels and ESD events are described. A protection circuit situated between the IO pins of the IC and the internal circuitry of the IC includes a voltage drop network and a plurality of shunt circuits to protect the IC against excessive supply voltages and ESD voltages, or other excessive current conditions. Each shunt circuit includes an RC trigger stage and an NMOS shunt stage that are made using low-voltage devices. A protection circuit of the embodiments includes a high voltage IO pin, a voltage drop network to drop a high voltage on the IO pin to a low voltage level on a floating voltage rail, a first shunt circuit coupled between the floating supply rail and ground, an equalizer circuit coupled between the floating supply rail and a low voltage supply rail, and a second shunt circuit coupled to the equalizer circuit through the low voltage supply rail.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to co-pending application entitled“Electrostatic Discharge Power Clamp Trigger Circuit Using Low StressVoltage Devices,” filed on Mar. 18, 2009, U.S. patent application Ser.No. 12/406,684, and assigned to the assignee of the present application.

TECHNICAL FIELD

The disclosed embodiments relate generally to integrated circuitdevices, and more specifically to high-voltage power supply and ESDprotection circuits.

BACKGROUND OF THE DISCLOSURE

Modern integrated circuits (ICs) have different types of input/output(IO) interfaces to communicate with other integrated circuits. Theinterfaces often have different power supply voltage levels, such as 5V,3.3V, 2.5V, 1.8V, and 1.2V to support a number of different peripheraldevices. This voltage range is broken down into two main categories,high voltage (2.5V-5V and higher), and low voltage (1.2V-1.8V). Mostmodern transistors, such as those made to present process scales on theorder of 45-65 nm processes, are capable of withstanding only lowvoltage supply levels (1.2V to 1.8V). Accommodating the entire possiblerange of supply voltages from high voltage (3.3V-5V) to low voltage(1.2V-1.8V) within a single IC represents a significant design andmanufacturing challenge, as such voltages must be properly distributedand buffered within the IC to ensure proper operation and protection ofthe transistors within the device.

An IC can be severely damaged or destroyed when subjected to a voltagethat is higher than the design voltage of the integrated circuit. Suchhigh voltages may be due to different power supply levels, or spuriouseffects, such as Electrostatic Discharge (ESD) events. In general,higher supply voltage levels require the use of thick gate oxide CMOStransistors, but lower stress voltage devices may still required to beused for such higher supply levels. For example, a 3.3V device can beused for a 5V IO interface, and a 2.5V, 1.8V or even 1.2V device can beused for a 3V or 2.5V IO interface. Thus, during normal operation, ICsshould be designed to accommodate relatively high supply voltage levels.

Besides potential high voltage exposure during normal operation, all ICsmust be protected from ESD effects, since the potential for exposure tosuch high voltage discharge is ever-present. ESD can originate fromsources such as storage bags, device carriers, machinery, host devices,and people. Such sources can easily generate a voltage that is manytimes greater than the design voltage of an IC. For example, the typicalhuman body can supply an electrostatic discharge of up to 6KV(kilovolts), as compared to the standard operating voltage for an IC of5V or less.

To protect the internal circuitry of an IC from high voltage or ESDevents, protection circuits are utilized, such as between the internalcircuitry and the IO pins of the IC. Present protection circuitstypically utilize reverse-biased diodes acting as avalanche breakdownclamps to limit the voltage between the power supply terminals of theIC. A problem associated with this approach is that the breakdownvoltage of the diode can vary widely depending on design and fabricationvariations. With advances in process technology, devices become eversmaller, and consequently, have ever lower electrostatic discharge (ESD)break down voltages.

Present IC circuits typically utilize shunt circuits that are isolatedfrom one another and configured to work with devices coupled to aparticular supply rail and voltage tolerance. The use and distributionof different supply voltage levels in a single IC, thus requires the useof several shunt circuits. It is desirable, therefore, to provide asupply voltage and ESD protection circuit that minimizes the number ofshunt devices required in IC circuits that have different types ofdevices and different power supply levels.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example and not limitation in thefigures of the accompanying drawings, in which like references indicatesimilar elements and in which:

FIG. 1 illustrates a block diagram of a high-voltage/ESD power clamptrigger circuit that uses low stress voltage devices under anembodiment.

FIG. 2 illustrates an example distribution of high voltage and lowvoltage devices in an IC with integrated protection circuitry, under anembodiment.

FIG. 3 is a circuit diagram of a high-voltage/ESD power clamp triggercircuit that uses low stress voltage devices, under an embodiment.

FIG. 4 illustrates a single-stage protection circuit using low-stressvoltage devices, under an embodiment.

DETAILED DESCRIPTION

Embodiments of the invention as described herein provide a solution tothe problems of conventional methods as stated above. In the followingdescription, various examples are given for illustration, but none areintended to be limiting.

Embodiments of an IC protection circuit that protects low voltage supplytransistors and circuits within the IC from excessive power supplylevels and ESD events are described. A protection circuit situatedbetween the IO pins of the IC and the internal circuitry of the ICincludes a voltage drop network and a plurality of shunt circuits toprotect the IC against excessive supply voltages and ESD voltages. Eachshunt circuit includes an RC trigger stage and an NMOS shunt stage thatare made using low-voltage devices. The shunt circuits are coupledthrough an equalization device that comprises a PMOS transistor placedin series between pairs of shunt circuits. The equalization circuitfunctions to hold floating rail voltage levels to lower voltage supplylevels.

FIG. 1 illustrates a block diagram of a high-voltage/ESD power clamptrigger circuit that uses low stress voltage devices under anembodiment. The circuit of FIG. 1 provides protection to internal ICcircuits from either or both of excessively high supply voltage levelsand ESD or similar voltage spike events. Circuit 100 comprises a voltagedrop network (VDN) 102 that is connected to one or more IO pins 110. TheIO pin 110 provides a high voltage supply level Vdd_hv 101, as well as apotential entry point for any ESD voltage that may be introduced to theIC. The voltage drop network 102 is coupled to a first shunt circuit104, and one or more additional shunt circuits 108-110 connected inseries. The two or more shunt circuits are coupled to one anotherthrough an equalizer circuit (or equalization device) 106.

It is assumed that the internal IC device circuits are configured tooperate at a relatively low voltage levels (e.g., 1.0V to 1.8V) Vdd_lv,as provided by the low voltage power supply rail 107.

The voltage drop network 102 provides a voltage drop from the highervoltage Vdd_hv to a voltage that is below the device stress voltage,Vdd_lv. The VDN can be implemented through a serial diode chain, orthrough diode-connected NMOS or PMOS devices, or through any othersimilar circuit. The shunt circuits 102 and 108 may be implementedthrough RC trigger circuits with NMOS shunt devices, or any similarcircuit, and are made using low-voltage circuits, also referred to as“low stress voltage” devices. The RC trigger circuits sense an ESD eventand turn on the NMOS ESD shunt devices to provide a current return pathto ground for the ESD current. The time constant of the RC network isdesigned to be on the order of 150 ns to 600 ns to provide enough timefor power clamp, but is not so limited.

In one embodiment, the equalizer circuit 106 is implemented as a PMOSdevice, but is not so limited. The equalizer circuit can be used whenthe Vdd_lv level 107 is the same as the device stress voltage. Itprovides an extra current path for both Vdd hv and Vdd_lv ESD event.This provides a higher ESD tolerance voltage and/or better ESD areaefficiency.

For circuit 100, a number of shunt circuits 104, 108, and 110 may becoupled in series through one or more equalizer circuits 106. Thetrigger circuits for the multiple shunt circuits work in tandem toprovide current paths to ground. Thus each additional shunt circuitprovides additional current shunt capability to circuit 100. Forexample, if two shunt circuits are employed, shunt circuit 104 may shunt80% of the current, and shunt circuit 108 may shunt the remaining 20% toground. The choice of devices and their relative distance are parametersthat can be used to dictate the relative amount of current dropped byeach shunt circuit. The number, N, of shunt circuits depends on theoperating characteristics of the IC circuit. For example, one to fiveshunt circuits can be used for high-stress level ICs and up to 20 or soshunt circuits can be used for low-stress level ICs.

As shown in FIG. 1, circuit 100 creates a floating rail (or supply) 105that is connected to the high voltage level Vdd_hv using the VDN, and tothe low voltage supply Vdd_lv. During normal operation, the floatingrail voltage will correspond to the Vdd_lv level, for example 1.8V.During an ESD event, however, the floating rail 105 is made tocorrespond to the Vdd_lv level through the combination voltage dropnetworks and shunt circuits.

The IO pin 110 may be connected to a high voltage supply that canprovide any number of high voltage levels, such as 2.5, 3.0, 3.3, 5.0Vand so on. Such high voltage levels are typically utilized by only asmall number of devices on the IC, with the majority of internalcircuits configured to operate at low voltage levels (e.g., 1.2-1.8V).In general, a typical IC may thus comprise on the order of 80%-90% lowvoltage devices, and only 10%-20% high voltage areas. FIG. 2 illustratesan example distribution of high voltage and low voltage devices in an ICwith integrated protection circuitry, under an embodiment. IC device 200comprises a number of circuits made up of low voltage transistors anddevices 202 and high voltage transistors and devices 204. The lowvoltage devices 202 operate using low voltage supply levels Vdd_lv, andthe high voltage devices operate using high voltage supply levelsVdd_hv. In a typical IC, the high voltage devices are located on theperiphery of the IC, and comprise a relatively small portion of the IC,such as on the order of 5-20%. However, the high voltage circuits mayalso be distributed throughout the IC and may comprise a higherproportion of the IC, depending on the actual IC design. For the systemof FIG. 2, the high voltage areas are associated with the IO regions ofthe IC and provide high voltage supply levels, as well as possible areasof entry for high voltage discharge from potential ESD sources. In oneembodiment, the one or more high voltage circuit areas 204 are coupledto the low voltage circuits 202 through one or more protection circuits206, as shown in FIG. 1. The protection circuits 206 protect the lowvoltage circuits 202 from potential ESD effects as well as the possiblyexcessive Vdd_hv voltage levels. In an embodiment, the protectioncircuits 206 themselves are made using low-voltage devices, as opposedto high-voltage devices.

In one embodiment, protection between the high voltage circuits and thelow voltage circuits is provided by a number of shunt circuits withineach protection circuit 206. As shown in FIG. 1, shunt circuit 104provides a shunt for the high voltage level Vdd_hv to ground, and shuntcircuit 108 provides a shunt for the low voltage level Vdd_lv to ground.Each shunt circuit may represent a number of shunt devices in parallel.The number of devices depends on the relative number of circuits toprotect. For example, for a typical IC as illustrated in FIG. 2 withroughly 10-20% high voltage circuits and 80-90% low voltage circuits,there may be on the order of five shunt devices in shunt circuit 104that are coupled in parallel to the floating rail 105, and up to 20 orso additional shunt devices in shunt circuit 108 that are coupled inparallel to the low voltage supply Vdd_lv.

FIG. 3 is a circuit diagram of a high-voltage/ESD power clamp triggercircuit comprising a protection circuit that uses low stress voltagedevices, under an embodiment. Circuit 300 illustrates one possibledevice-level implementation of the protection circuit 100 illustrated inFIG. 1. The high voltage supply level 301 is input to a voltage dropnetwork (VDN) 304. For the embodiment of FIG. 3, VDN 304 is implementedas a number forward-drop diodes coupled in series. The number of diodesdepends on the voltage level of Vdd_hv and the low voltage level ofVdd_lv, 303. The lower node of VDN 304 is coupled to floating rail 305.The floating rail 305 is coupled to a high voltage shunt circuit thatcomprises an RC trigger stage 306 and an NMOS shunt transistor 312.Transistor 312 forms the shunt device portion of shunt circuit 104 inFIG. 1.

For the embodiment of FIG. 3, the trigger portion 306 of the shuntcircuit is an RC trigger, though any similar type of trigger circuit maybe used. RC trigger circuit 306 comprises a resistor (R) coupled inseries to a capacitor (C), which are connected to an ESD triggerinverter. The RC trigger circuit 306 takes advantage of the fact that itis possible to distinguish between an ESD event and a normal applicationof power by a difference in rise time. For example, an ESD event mayresult in a rise time on a power supply line in the range of 10nanoseconds, whereas the rise time during regular application of powerto the supply line may be on the order of greater than 1 microsecond.The ESD shunt circuit 100 makes use of an RC time constant produced by aseries configuration of the trigger capacitor and the trigger resistor.The RC time constant is selected such that it is shorter than amagnitude of a rise time expected on a power supply node and alsosufficiently long enough to provide full dissipation of a charge buildup from an ESD event prior to turning off a shunt. The time required todischarge the ESD event is dependent on the time constant determined bya discharging network and the RC time constant of the trigger device.The ESD shunt trigger line from the trigger inverter is coupled to theNMOS shunt transistor 312, which shunts the ESD current to ground orsource voltage Vss.

A second RC trigger 308 and NMOS shunt 314 circuit are coupled betweenthe low voltage supply rail Vdd_lv 303 and ground. The first and secondshunt circuits are coupled to each other through an equalization device310, which in one embodiment is implemented as a single PMOS transistor,as shown in circuit 300. The NMOS ESD shunt devices 312 and 314 providethe power clamp from Vdd_hv to Vss and Vdd_lv to Vss during ESD event toprotect the IC from ESD damage.

During normal operation (non ESD protection mode) the high voltage levelVdd_hv 301 from the high voltage portion of the IC is applied to theprotection circuit. In certain cases, this voltage may be excessivelyhigh and cause an overstress voltage for the low voltage portions of theIC. The protection circuit 300 protects the IC from any such overstresscaused by the high voltage sources. The Vdd_hv voltage will go throughthe forward-biased diodes of the VDN 304 that drops the voltage fromVdd_hv to the V_(drop) level. In one embodiment, the V_(drop) value isdictated by the following two equations:

V_(drop)=Vdd_hv−ΔV   (1)

V_(drop)<Vdd_lv   (2)

In the above equations, ΔV is the number of diodes in the VDN 304. TheVDN 304 is configured to drop the V_(drop) voltage to below the deviceoverstress voltage and the Vdd_lv value. Thus, the circuits of the ICwill be protected from voltage overstress. If an equalization device ispresent, the V_(drop) level will be pulled to the low voltage railVdd_lv 303. Thus, during normal operation the equalization device 310 isused to hold the V_(drop) level to Vdd_lv to avoid a lower voltage onV_(drop).

In general, the equalization device 310 performs two distinctoperations. During normal operation of the IC device, it holds theV_(drop) voltage level to the Vdd_lv level, and during an ESD event, theequalization device provides an additional current path to improve ESDprotection.

During an ESD event, the IC device is not powered up, so the Vdd_hv andVdd_lv rails to not carry their normal operating currents. Instead theyprovide pathways for any ESD current. Thus, the voltage spike currenttypically travels through the Vdd_hv rail 301, and possibly, the Vdd_lvrail to the internal IC circuitry. In circuit 300, the ESD current willrun through the VDN forward-biased diodes 304 and trigger the RC triggercircuit 306 to turn on the NMOS shunt 312 in the first shunt device.Part of the ESD current will also go through the equalization device 310to trigger the Vdd_lv RC trigger circuit 308 to turn on NMOS shunt 314.In this manner, circuit 300 provides higher ESD protection voltage orbetter area ESD protection efficiency than current protection circuits,such as NMOS snapback devices.

During an ESD event, the shunt 312 provides the Vdd_hv to ground path todischarge the ESD energy. When ESD current is discharged through the lowvoltage rail Vdd_lv 303, the low voltage shunt 314 and RC trigger 308provides an ESD discharge path from Vdd_lv to ground. The reverse-biaseddiodes in VDN 304 will block the current path from Vdd_lv to Vdd_hv.This also prevents the need from the ESD structure requiring a power-upsequence. In ESD protection mode, the equalization device 310 providesan additional ESD current path through the Vdd_lv ESD shunt 314 for ESDcurrent from Vdd_hv, or to the Vdrop shunt 312 for ESD current fromVdd_lv.

The equalization device 310 thus allows the high voltage shunt circuit312 and the low voltage shunt circuit 314 to work together to dissipatethe ESD current. As described above with reference to FIGS. 1 and 2, anumber of low voltage and high voltage shunt devices may be used inparallel to expand the current dissipating capacity of the protectioncircuit. In this case, there would be a number of RC trigger circuits306 and high voltage shunts 312 connected in parallel depending on therelative amount of high voltage devices in the IC, and a second numberof RC trigger circuits 308 and low voltage shunts 314 connected inparallel depending on the relative amount of low voltage devices in theIC. In a typical IC, there would usually be on the order of four to fivetimes as many low voltage trigger and shunt devices than high voltagetrigger and shunt devices, depending on the relative proportion of highand low voltage devices in the IC. The use of an equalization device 310coupling the high and low voltage shunt circuits to one anothereliminates the need to grow the circuit size of the IC to provide extraprotection for the high voltage devices. That is, through the use ofhigh and low voltage shunts coupled through an equalizer device, theprotection circuit 300 can be configured to provide ESD protection topractically any number of low voltage and high voltage devices withoutrequiring extensive growth of the IC.

Embodiments of the protection circuit 300 include an individual shuntcircuit that works in conjunction with a voltage drop network toefficiently provide ESD protection against voltage spikes through an IOpin. FIG. 4 illustrates a single-stage ESD protection circuit forprotecting low voltage devices from ESD events on a high voltage 10 pin,under an embodiment. As shown in circuit 400 a high voltage IO pin 401is coupled to a voltage drop network 402 that comprises a number offorward-bias diodes connected in series. These diodes drop the voltagefrom the Vdd_hv level to a lower voltage level (Vdrop) 403, depending onthe number of diodes in the VDN 402. An RC trigger circuit 406consisting of a resistor 410 in series with capacitor 412 and coupled toa trigger inverter 408 provides a trigger control signal that activatesNMOS shunt transistor 404. Circuit 400 provides an efficient means ofprotecting low voltage circuits from ESD effects that may be introducedon high voltage IO pins. It offers benefits over present NMOS snapbacksystems in that it is scalable to lower device sizes and enables the useof RC shunts in conjunction with high voltages (e.g., 5V) using lowstress voltage devices. Embodiments described herein are directed to acircuit for protecting low voltage devices within an integrated circuit(IC) from electrostatic discharge (ESD) events, comprising: 1.

A circuit for protecting low voltage devices within an integratedcircuit (IC) from excessively high voltage supply levels, comprising: ahigh voltage input/output (IO) pin, a floating voltage rail, a firstshunt circuit coupled between the floating supply rail and ground, anequalizer circuit coupled between the floating supply rail and a lowvoltage supply rail, and a second shunt circuit coupled to the equalizercircuit through the low voltage supply rail. In this circuit, the highvoltage level may be within the range of 3.3V to 5V, and the low voltagelevel may be within the range of 1.2V to 1.8V. In an embodiment, thevoltage drop network comprises a plurality of forward-biased diodesconnected in series. In this circuit, the equalizer circuit may comprisea PMOS transistor.

Each of the first shunt circuit and second shunt circuit may comprise:an NMOS shunt transistor coupled between a respective voltage supplyrail and ground, and a trigger circuit coupled to the NMOS shunttransistor to activate the shunt transistor when a sensed input voltagerise time is shorter than a defined supply voltage rise time. Thetrigger circuit may be a resistor-capacitor (RC) trigger circuitcomprising a capacitor coupled in series to a resistor and a triggerinverter. In this trigger circuit, the resistor and capacitor values areselected to produce an RC time constant that is shorter than an expectedrise time for the respective supply voltage rail, and long enough toprovide full dissipation of a charge build up from the ESD event priorto activation of the NMOS shunt transistor.

The systems and/or components described herein may be implemented as oneor more electronic circuits. Such circuits described herein can beimplemented through the control of manufacturing processes andmaskworks, which would be then used to manufacture the relevantcircuitry. Such manufacturing process control and maskwork generationknown to those of ordinary skill in the art include the storage ofcomputer instructions on computer readable media including, for example,Verilog, VHDL or instructions in other hardware description languages.

Aspects of the system described herein may be implemented as hardwarecircuitry involving several different device processes. The underlyingdevice technologies may be provided in a variety of component types,e.g., metal-oxide semiconductor field-effect transistor (“MOSFET”)technologies like complementary metal-oxide semiconductor (“CMOS”),bipolar technologies like emitter-coupled logic (“ECL”), polymertechnologies (e.g., silicon-conjugated polymer and metal-conjugatedpolymer-metal structures), mixed analog and digital, and so on.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in a sense of “including,but not limited to.” Words using the singular or plural number alsoinclude the plural or singular number respectively. Additionally, thewords “herein,” “hereunder,” “above,” “below,” and words of similarimport refer to this application as a whole and not to any particularportions of this application. When the word “or” is used in reference toa list of two or more items, that word covers all of the followinginterpretations of the word: any of the items in the list, all of theitems in the list and any combination of the items in the list.

The above description of illustrated embodiments of the IC protectioncircuit is not intended to be exhaustive or to limit the embodiments tothe precise form or instructions disclosed. While specific embodimentsof, and examples for, circuits and components are described herein forillustrative purposes, various equivalent modifications are possiblewithin the scope of the disclosed methods and structures, as thoseskilled in the relevant art will recognize.

The elements and acts of the various embodiments described above can becombined to provide further embodiments. These and other changes can bemade to the disclosed system in light of the above detailed description.

In general, in the following claims, the terms used should not beconstrued to limit the disclosed method to the specific embodimentsdisclosed in the specification and the claims, but should be construedto include all operations or processes that operate under the claims.Accordingly, the disclosed structures and methods are not limited by thedisclosure, but instead the scope of the recited method is to bedetermined entirely by the claims.

While certain aspects of the disclosed embodiments are presented belowin certain claim forms, the inventors contemplate the various aspects ofthe methodology in any number of claim forms. Accordingly, the inventorreserves the right to add additional claims after filing the applicationto pursue such additional claim forms for other aspects.

1. A circuit for protecting low voltage devices within an integrated circuit (IC) from excessively high power supply levels, comprising: a high voltage input/output (10) pin; a floating voltage rail coupled to the high voltage IO pin; a first shunt circuit coupled between the floating supply rail and ground; an equalizer circuit coupled between the floating supply rail and a low voltage supply rail; and a second shunt circuit coupled to the equalizer circuit through the low voltage supply rail.
 2. The circuit of claim 1 wherein the high voltage level is a voltage within the range of 3.3V to 5V, and wherein the low voltage level is a voltage within the range of 1.2V to 1.8V.
 3. The circuit of claim 1 wherein the equalizer circuit holds the floating supply rail voltage level to the low voltage supply rail voltage level during normal operation of the IC.
 4. The circuit of claim 3 wherein the equalizer circuit comprises a PMOS transistor.
 5. The circuit of claim 1 wherein the components of the equalizer circuit, the first shunt circuit and the second shunt circuit are configured to operate only at a low voltage level provide by the low voltage supply rail.
 6. The circuit of claim 5 wherein the IC comprises a first portion consisting of low voltage devices powered by the low voltage supply rail, and a second portion consisting of high voltage devices powered by the high voltage supply rail.
 7. The circuit of claim 6 wherein the first shunt circuit comprises a first plurality of RC trigger circuits and NMOS shunt transistors coupled in parallel and wherein the first plurality depends on the number of high voltage devices in the second portion.
 8. The circuit of claim 7 wherein the second shunt circuit comprises a second plurality of RC trigger circuits and NMOS shunt transistors coupled in parallel and wherein the second plurality depends on the number of low voltage devices in the first portion.
 9. The circuit of claim 1 further comprising a voltage drop network coupled to the IO pin to drop a high voltage on the IO pin to a low voltage level on the floating voltage rail, and wherein each of the first shunt circuit and second shunt circuit includes an NMOS shunt transistor coupled between a respective voltage supply rail and ground, and a trigger circuit coupled to the NMOS shunt transistor to activate the shunt transistor when a sensed input voltage rise time is shorter than a defined supply voltage rise time.
 10. The circuit of claim 9 wherein the trigger circuit is a resistor-capacitor (RC) trigger circuit comprising a capacitor coupled in series to a resistor and a trigger inverter.
 11. A voltage equalization circuit for an integrated circuit (IC) composed of high voltage devices and low voltage devices, the circuit comprising: a voltage drop network dropping a high voltage generated by a high voltage power supply on the IC device to a lower voltage level on a floating supply rail; a parallel array of first shunt circuits coupled between the floating supply rail and ground; a parallel array of second shunt circuits coupled between a low voltage rail supplying power to the low voltage devices and ground; and an equalizer circuit coupled between the parallel array of first shunt circuits and the parallel array of second shunt circuits.
 12. The circuit of claim 11 wherein the equalizer circuit causes a voltage level on the floating supply rail to conform to the low voltage level supplied on the low voltage rail.
 13. The circuit of claim 12 wherein the equalizer circuit comprises a PMOS transistor.
 14. The circuit of claim 12 wherein the parallel array of first shunt circuits comprises a number of shunt circuits corresponding to a relative number of high voltage devices in the IC.
 15. The circuit of claim 14 wherein the parallel array of second shunt circuits comprises a number of shunt circuits corresponding to a relative number of low voltage devices in the IC.
 16. The circuit of claim 11 wherein the voltage drop network comprises a plurality of forward-biased diodes connected in series.
 17. The circuit of claim 11 wherein the parallel array of first shunt circuits shunts excessive current incident in IC via a high voltage input pin to ground through a first discharge path, and wherein the parallel array of second shunt circuits shunts the excessive current to ground through a second discharge path.
 18. The circuit of claim 17 wherein each shunt circuit of the first and second shunt circuits comprises a resistor-capacitor (RC) trigger circuit coupled an NMOS shunt transistor.
 19. A computer readable media storing instructions thereon wherein said instructions are adapted to control a manufacturing process to manufacture a circuit comprising: a high voltage input/output (IO) pin; a floating voltage rail coupled to the high voltage IO pin; a first shunt circuit coupled between the floating supply rail and ground; an equalizer circuit coupled between the floating supply rail and a low voltage supply rail; and a second shunt circuit coupled to the equalizer circuit through the low voltage supply rail.
 20. The computer readable media of claim 19 wherein said instructions comprise hardware description language instructions. 